Leafminers

Leafminer adults are small, black flies about 0.06 inches (1.5 mm) long, with a bright yellow triangle on the thorax between the base of the wings. They are yellow on the face and underside. Females puncture leaves to feed on plant sap and lay eggs within the leaf tissue. Eggs hatch in 2 to 4 days during warm weather. The yellow maggots (larvae) grow to be up to 2.5 mm long as they feed between the upper and lower surface of leaves. Feeding by the maggots causes distinctive winding, whitish tunnels that increase in length and width as the larva grows. Mature maggots emerge to pupate on the leaf surface or drop into the soil to pupate. Many generations occur each year; the entire life cycle can be completed in less than 3 weeks when the weather is warm.

The species can be distinguished by the adult's appearance or by the appearance of the mines.

The serpentine leafminer is most common in the warmer, celery-growing areas of Southern California and the Central Valley; it does not occur as readily in the cooler growing areas of the Central Coast. It is the smallest and most yellow of the three and produces characteristically wandering mines.

The pea leafminer occurs in the Salinas Valley as well as in the Central Valley, where it occurs with the serpentine leafminer. It also appears to be extending its distribution into the southern growing regions of the state. The pea leafminer is the largest and most black or gray and, following the first two instars, produces straight mines. It is more likely than the other species to mine the petioles and stalks of celery.

The vegetable leafminer may be present in the coastal areas in low numbers.

Leafminers can reduce the plant’s photosynthetic capacity, render edible portions of the plant unmarketable, and provide an entrance for pathogens. Many countries and some states regulate the importation of produce grown in areas where pea leafminer exists to prevent its introduction.

Natural enemies, especially parasitoid wasps in the genus Diglyphus, commonly reduce numbers of leafminers, unless they are killed by insecticides applied to control other pests. Several other parasites also attack leafminers.

Predators are not important biological control agents because leafminer eggs and larvae are protected within leaf tissue.

Choose selective pesticides when managing other pests to avoid disrupting leafminer biological control, killing parasites, and inducing leafminer outbreaks. Consider installing insectary plants to attract natural enemies of leafminers.

Avoid planting next to infested fields, especially lettuce fields near harvest, because leafminers attack a wide variety of vegetable crops that are often grown in proximity to celery.

Use cultural controls, biological controls, and sprays of azadirachtin in a certified organic crop.

Treatment thresholds must be based on the species involved. To measure the severity of the infestation of the serpentine leafminer, place trays between the rows of celery to catch dropping pupae. Trays about 4 inches by 9 inches fit well between the rows. Consider applying an insecticide if there are catches of 15 pupae or more per day per tray.

Populations of pea leafminer are not as easily monitored. The treatment threshold is probably considerably lower than for the serpentine and vegetable leafminers because of the tendency of the pea leafminer to mine the stalks.